1. Field of the Invention
This invention concerns improvements in and relating to investigating current, particularly but no exclusively in relation to investigating the current actually passing through an area under going field signature method based investigations.
2. Related Technology and Summary of the Invention
The field signature method is based upon feeding a direct current through a location and measuring the electric field which is generated as a result using an array of electrical contacts on a surface of the location. Changes in the magnitude and shape of the electric field overtime can provide significant information on corrosion occurring at the location.
To make measurements an excitation current has to be applied to the location. The voltage across one or more pairs of contacts on the location to be measured and the voltage across one or more pairs of contacts on a non-corroding reference are measured repeatedly to monitor the corrosion that might be occurring.
Such a system works well in many location configurations. The present applicant has found, however, that most real life situations are unsuited for investigation using the prior art technology. Signal instability effects in those cases swamps any variation arising from the corrosion progressing. This represents a significant limitation on the applicability of such techniques.
Through investigation and detailed consideration of these real life situations the applicant has realised that only a proportion of the excitation current applied will pass through the location, with the remainder finding other paths to complete the circuit. Part of the excitation current will pass through structural supports and other such locations surrounding the location, for instance. Furthermore, the proportion of the split going through the location is very likely to vary overtime due to events having an effect on the resistance offered by the alternative routes and/or the location itself. Thus corrosion of the other routes, changes to the support structure or the like could cause variations which swamp the electric field changes caused by corrosion in the location.
The present invention aims to provide for wider applicability of field signature based techniques by identifying the source of the problem and through the provision of a method which offers a clearer picture of corrosion arising, amongst other aims.
According to a first aspect of the invention we provide a method of investigating corrosion at a location, the method including
measuring the variation in the voltage between two or more electrical contacts at a first time and at one or more other times, the two or more electrical contacts being in contact with the location,
passing a current through the location at the time of the voltage measurements,
the current passing through the location at one or more of voltage measurements times being measured.
Preferably the method includes providing a power source external of the location to provide an applied current.
Preferably the method provides for using the respective voltage values from the two or more electrical contacts in the investigation of the corrosion.
The method may provide for the part of the applied current passing through the location being measured by the use of non-intrusive means, preferably the Hall effect. The non-intrusive means of measuring may be a bismuth spiral, a magnetically sensitive F.E.T., a rotating coil, deflection of a moveable ion vein or the use of the magnetoresistive effect.
According to a second aspect of the invention we provide a method of investigating corrosion at a location, the method including
measuring the variation in the voltage between two or more electrical contacts at a first time and at one or more other times, the two or more electrical contacts being in contact with the location,
passing a current through the location at the time of the voltage measurements,
providing a power source external of the location to provide an applied current,
using the respective voltage values from the two or more electrical contacts in the investigation of the corrosion,
the part of the applied current passing through the location being measured by the use of the Hall effect.
The first and/or second aspects of the invention may involve one or more of the following features, options or possibilities.
The corrosion investigated may occur throughout the location and/or at one or more specific parts of the location. The corrosion may occur due to contact between the location and its environment. The environment in question may be the external environment for the location and/or the internal environment for the location. The corrosion may arise as a result of chemical attack on the location and/or mechanical wear on the location.
The location may be an entire article or a part there of. The location may particularly be a part of a chemical plant. The location may be a part or the whole of a pipeline, passageway, conduit, vessel, container, wall or barrier. The location may have one or more surfaces isolated from one or more other surfaces. The corrosion may effect one or more of the sets of isolated surfaces, for instance the internal surfaces in the case of a pipeline.
The variation in voltage may be an increase in voltage as corrosion progresses. The variation may occur evenly for all the respective electrical contacts considered. The variation may occur unevenly for all the respective electrical contacts considered. The variation may occur at an even rate over time. The variation may occur at an uneven rate over time. The variation may occur at an even rate around the cross-section of a pipeline or other conduit. The variation may occur at an uneven rate around the cross-section of a pipeline or other conduit.
The electrical contacts may be provided by pins or other electrically conducting elements. Preferably the electrical contacts are resiliently forced into contact with the location, for instance by springs. Ideally the electrical contacts are welded or otherwise fixed to the location. The electrical contacts may be provided in pairs, preferably with the voltage between predefined pairs being measured during the investigation. The electrical contacts and/or pairs of electrical contacts may be evenly spaced along the direction of current flow or unevenly spaced along the direction of current flow. Electrical contacts may be provided throughout the location, in the direction of current flow and/or perpendicular to the direction of current flow. The electrical contacts may be provided all around the cross-section of a pipeline or other form of conduit. The method may involve measuring the voltage for one or more pairs of electrical contacts simultaneously. Four or more and preferably eight or more pairs may be considered simultaneously. The number of pins provided may be between 8 and 256 pins, more preferably between 16 and 128 pins and ideally between 24 and 64 pins.
The electrical contacts may be provided by an electrical contact mounting unit. The mounting unit is preferably configured to match one or more surfaces of the location to be investigated. The mounting unit may be in the form of a ring, preferably a breakable ring, to investigate pipelines, conduits or other locations of circular or partially circular cross-section. The mounting unit may be in the form of a collar. Preferably all the electrical contacts are provided on a single mounting unit.
The current is preferably a DC current and particularly a square wave DC current. The DC current may be provided in a single direction but is more preferably applied in both directions, ideally alternately. The current may be applied for between 200 and 2000 ms−1 per time and more preferably between 500 and 1000 ms−1.
The current may be introduced to the location at one end thereof and leave at the other end thereof. The current may be introduced and/or exit by a current contact unit, preferably configured to match one or more surfaces of the location to be investigated or an element in electrical contact therewith. The current contact unit or units may be in the form of a ring, preferably a breakable ring, to investigate pipelines, conduits or other locations of circular or partially circular cross-section. The current contact unit or units may be in the form of a collar.
The voltage measurements may be made after the current has started. Preferably the voltage measurements are made at least 200 ms−1 after the current has been applied. Preferably the voltage measurement is made within 800 ms−1 of the current being applied. Preferably the voltage measurements are made after the current stops preferentially flowing in the surface part of the location. Preferably the voltage is steady when the voltage measurements are made.
Preferably the temperature of the two or more electrical contacts is measured at one or more of the first time and one or more other times. Preferably the temperature is measured each time a voltage is measured. Preferably the temperature of the current measuring means, for instance the non-intrusive means, is measured at one or more of the first time and one or more other times. Preferably the temperature of the current measuring means is measured at each current measurement. The temperature of the two or more electrical contacts and/or current measuring means may be measured by measuring the temperature of the electrical contacts and/or current measuring means. The temperature of the two or more electrical contacts and/or current measuring means may be measured by measuring the temperature of the location. The temperature of the two or more electrical contacts and/or current measuring means may be measured by measuring the temperature of the environment surrounding the two or more electrical contacts and/or current measuring means.
Preferably the voltage measurements are compensated for temperature variations at the electrical contacts and/or location and/or current measuring means and/or the environment(s) thereof.
The power source may be a mains power source or portable power source, such as a battery. The power source may provide the same or a different current level for respective measurements.
The current for the location at a measurement time may be used to compensate one or more voltages measured for the location or a part thereof at that measurement time for variations in the current at that measurement time compared with one or more other measurement times. Preferably the compensation is made according to the equation:       Fc    Ai    =                              I          s                          A          s                    ×                        A          i                          I          i                      -          1      ×      1000      ⁢                           ⁢              (                  parts          ⁢                                           ⁢          per          ⁢                                           ⁢          thousand                )            where                FcAi=fingerprint coefficient for a pair of electrical contacts A under investigation at time i;        As=voltage across pair A at a reference time, preferably the start;        Is=current passing through location at the reference time, preferably the start;        Ai=voltage across pair A at time i;        Bi=current passing through location at the time i.        
The current measurement may be made at the voltage measurement location, for instance by providing the current measuring means as part of the mounting unit. The current measuring means may be provided at a different part of the location to the part where voltage measurements are made. The current may pass through the current measuring location prior to passing through the voltage measuring location, the current measuring location being provided between the closest potential current leakage route and the voltage measuring location considered in the direction of current flow. The current may pass through the voltage measuring location prior to passing through the current measuring location, the current measuring location being provided between the voltage measuring location and the closest potential current leakage route considered in the direction of current flow.
The Hall effect may be measured to measure the current passing though the location. The Hall effect measurement may be taken to be proportional to the current passing, preferably linearly proportional to the current passing through the location. The Hall effect may be measured by providing a semi-conductor material and applying a constant current through it in a first direction. The first direction is preferably configured to be substantially, and ideally to be, perpendicular to the direction of the magnetic field, the second direction, generated by the current passing through the location. The Hall effect may be measured by measuring the Hall voltage. The Hall effect may be measured by measuring a voltage arising across the semi-conductor, preferably substantially perpendicular to the first and the second direction and ideally perpendicular to both directions.
The Hall effect may be measured using a Hall effect transducer.
The Hall effect transducer may be configured to match one or more surfaces of the location. The Hall effect transducer may be in the form of a ring, preferably a breakable ring, to investigate pipelines, conduits or other locations of circular or partially circular cross-section. The Hall effect transducer may be in the form of a collar. The Hall effect transducer is preferably provided in proximity with the location. The Hall effect transducer is preferably electrically isolated from the current passing through the location.
The Hall effect transducer may include a semi-conductor material and ferromagnetic material, the ferromagnetic material being provided between at least part of the semi-conductor material and the location. Preferably an air gap is provided between the ferromagnetic material and the surface of the location. A material for reflecting thermal radiation may be provided between the ferromagnetic material and the location, for instance a reflective foil. The ferromagnetic material may be of the ferrite Mn—Zn variety or of the ferrite Si—Fe variety.
The measurements made by the Hall effect transducer may be corrected for variations in temperature. A temperature sensor in proximity with the Hall effect transducer may be used to effect the temperature correction, for instance via a feedback circuit which is used to maintain the drive current to the Hall effect transducer at a value which nullifies variations in temperature over time.
The Hall effect transducer, and particularly the semi-conductor material thereof, may be shielded against radiation, particularly radiation arising within the location being considered. High density materials such as tungsten, lead, depleted uranium or other heavy alloys may be used for this purpose.
Preferably the current is measured by use of the Hall effect at substantially the same time, and ideally at the same time, as one or more of the voltage measurements are made. Preferably the current is measured by use of the Hall effect whenever a voltage measurement is made. The current may be measured when the voltage between the electrical contacts being considered is stable and/or constant. Preferably the current measurement is made at least 200 ms−1 after the current has been applied. Preferably the current measurement is made within 800 ms−1 of the current being applied. Preferably the current measurements are made after the current stops preferentially flowing in the surface part of the location.
The method preferably includes compensating voltage measurements at one or more times relative to voltage measurements at one or more other times according to any variation in the proportion of the applied current which passes through the location at different times of investigation.
According to a third aspect of the invention we provide apparatus for investigating corrosion at a location, the apparatus including
two or more electrical contacts in contact with the location in use,
means for measuring the variation in the voltage between the two or more electrical contacts, at a first time and at one or more other times,
a power source for passing a current through the location at the time of the voltage measurements,
current measuring means for the current passing through the location at one or more of the voltage measurements times, the current measuring means being non-intrusive.
According to a fourth aspect of the invention we provide apparatus for investigating corrosion at a location, the apparatus including
two or more electrical contacts in contact with the location in use,
means for measuring the variation in the voltage between the two or more electrical contacts at a first time and at one or more other times,
a power source to provide an applied current,
means for passing a current through the location at the time of the voltage measurements,
passing the applied current through a reference location and measuring the variation in the voltage between two or more reference electrical contacts at the first time and at one or more of the one or more other times, the two or more reference electrical contacts being in contact with the reference location,
using the respective voltage values from the two or more electrical contacts and two or more reference electrical contacts in the investigation of the corrosion,
the part of the applied current passing through the location being measured by the use of the Hall effect.
The third and/or fourth aspects of the invention may include any of the features, options or possibilities set out elsewhere in this document, including means to implement them.
The electrical contacts may be provided by an electrical contact mounting unit. A mounting unit is preferably configured to match one or more surfaces of the location to be investigated. The mounting unit may be in the form of a ring, preferably a breakable ring. The mounting unit may be in the form of a partial ring. The mounting unit may be in the form of a collar.
The electrical contact may be provided by pins or other electrically conducting elements, preferably mounted on the mounting unit. Preferably the electrical contacts are resiliently biased, ideally towards the location in use. The electrical contact may be biased by springs. The electrical contacts are preferably provided in pairs. The electrical contact and/or pairs of electrical contacts may be evenly spaced along the mounting unit.
Preferably the current is introduced by an external power source. A mains power source or portable power source, such as a battery, may be used.
The apparatus may include beams for introducing the current to one part of the location and removing the current from another part of the location. The means may comprise a current contact unit, preferably configured to match one or more surfaces of the location to be investigated and/or an element in electrical contact with one or more surfaces of the location to be investigated. The current contact unit or units may be in the form of a ring, preferably a breakable ring. The current contact unit or units may be in the form of a partial ring and/or collar.
The non-intrusive current measuring means may be configured to match one or more surfaces of the location. The non-intrusive current measuring means may be in the form of a ring, preferably a breakable ring. The non-intrusive current measuring means may be in the form of a collar and/or partial ring.
The Hall effect current measuring means may be provided by a semi-conductor material with an applied constant current passing through it in a first direction. The first direction is preferably configured to be substantially, and ideally to be, perpendicular to the direction of the magnetic field, the second direction, generated by the current passing through the location. The Hall effect may be measured by measuring the Hall voltage. The Hall effect may be measured by measuring a voltage arising across the semi-conductor, preferably substantially perpendicular to the first and second direction, ideally perpendicular to both directions. The Hall effect current measuring means may be one or more Hall effect transducers.